13.1.6 Factors Affecting Colour

• The size of the splitting energy ΔE in the d-orbitals is influenced by the following four factors:
  • The size and type of ligands
  • The nuclear charge and identity of the metal ion
  • The oxidation state of the metal
  • The shape of the complex

The large variety of coloured compounds is a defining characteristic of transition metals

Size and type of ligand

• The nature of the ligand influences the strength of the interaction between ligand and central metal ion
• Ligands vary in their charge density
• The greater the charge density; the more strongly the ligand interacts with the metal ion causing greater splitting of the d-orbitals
• The further it is then shifted towards the region of the spectrum where it absorbs higher energy
• As a result, a different colour of light is absorbed by the complex solution and a different complementary colour is observed
• This means that complexes with the same transition elements ions, but different ligands, can have different colours
  • For example, the [Cu(H₂O)₆]²⁺ complex has a light blue colour
  • Whereas the [Cu(NH₃)₄ (H₂O)₂]²⁺ has a dark blue colour despite the copper(II) ion having an oxidation state of +2 in both complexes

Ligand exchange of the water ligands by ammonia ligands causes a change in colour of the copper(II) complex solution

• Ammonia has a greater charge density than water and so produces a larger split in the d-orbitals

Graph showing the replacement of the water molecules with four ammonia molecules causes a shift in maximum absorbance towards shorter wavelength

The nuclear charge

• The strength of the attraction between the metal ion and lone pairs of electrons from the ligand can vary depending on the effective nuclear charge on the metal ion
• For example, aqueous Mn(II) and Fe(III) have the same electronic configuration:

[Ar] 3d⁵

• Mn(II) (Z=25) absorbs in the green region of the spectrum so appears pink
• The higher effective nuclear charge on Fe(III) (Z= 26) causes a stronger interaction with the ligands, so it absorbs in the higher energy blue part of the spectrum and appears yellow/orange in colour

Oxidation state

• When the same metal is in a higher oxidation state that will also create a stronger interaction with the ligands
• If you compare iron(II) and iron (III):
  • [Fe(H₂O)₆]²⁺ absorbs in the red region and appears green
  • But, [Fe(H₂O)₆]³⁺ absorbs in blue region and appears orange

Shape

• The change of colour in a complex is also partly due to the change in coordination number and geometry of the complex ion
• The splitting energy, ΔE, of the d-orbitals is affected by the relative orientation of the ligand as well as the d-orbitals
• Changing the coordination number generally involves changing the ligand as well, so it is a combination of these factors that alters the strength of the interactions

• The Japanese chemist, R. Tuschida, proposed a ranking of ligands base on their ability to separate the two sets of d-orbitals
• This is known as the spectrochemical series
• The higher the charge density of the ligand; the more strongly it influences the splitting of the d-orbitals so the greater the energy difference between them

Table showing the spectrochemical series for common ligands

• You can see at either end of the series lies iodide ions, I^-, and carbon monoxide, CO
• Iodide ions are large (think how many electron shells they have) so they have a relative low charge density and produce the weakest electric field so the separation energy of the d-orbitals is the smallest in the series
• Chloride ions, Cl^-, on the other hand are smaller, have a higher charge density and consequently produce a large separation energy
• However, size is not the only factor as carbon monoxide and cyanide ions produce a larger splitting due to complex interactions involving the pi bonds present in those molecules; those interactions occur when electrons in the d-orbitals of the transition metal interact with electrons in the p-orbitals of the ligands
• You should be able to see why adding ammonia to aqueous copper(II) ions results in a darker blue complex
  • Ammonia is a stronger ligand than water so the separation energy is larger and the wavelength of absorbed light shorter
  • Shorter wavelength means moving towards to bluer higher energy end of the visible spectrum